Buffering effects on the solution behavior and hydrolytic degradation of poly(β-amino ester)s

With a vast, synthetically accessible compositional space and highly tunable hydrolysis rates, poly(β-amino ester)s (PBAEs) are an attractive degradable polymer platform. Leveraging PBAEs in a wide range of applications hinges on the ability to program degradation, which, thus far, has been frustrated by multiple confounding phenomena contributing to the degradation of these charged polyesters. Basic conditions accelerate hydrolysis, yet reduce solubility, limiting water access to amines and esters. Further, the high buffering capacity of PBAEs can render buffers ineffective at controlling solution pH. To unify understanding of PBAE degradation and solution properties, this study examines PBAE hydrolysis as a function of pH and buffer concentration as well as polymer hydrophobicity. At low buffer concentrations, the PBAE amines and the acid produced during hydrolysis control solution pH. Meanwhile, at high buffer concentrations that afford relatively constant pH, hydrolysis rate increases with pH, despite the reduced PBAE solubility. Increasing the hydrophobic content of PBAEs eventually hinders the capacity of the polymer to accept protons from solution, limiting the pH increase and slowing hydrolysis. These studies showcase the role of buffering on the pH-dependent degradation and solution properties of PBAEs, providing guidance for programming degradation in applications ranging from drug delivery to thermosets.     

On the Snider equation

We study the physical content of the Snider quantum transport equation and the origin of a puzzling feature of this equation, which implies contradictory values for the one-particle density operator. We discuss in detail why the two values are in fact not very different provided that the studied particles have sufficiently large wave packets and only a small interaction probability, a condition which puts a limit on the validity of the Snider equation. In order to improve its range of application, we propose a reinterpretation of the equation as a mixed equation relating the real one-particle distribution function (on the left-hand side of the equation) to the free distribution (on the right-hand side), which we have introduced in a recent contribution. In its original form, the Snider equation is valid only when used to generate Boltzmann-type equations where collisions are treated as point processes in space and time (no range, no duration); in this approximation, virial corrections are not included, so that the real and free distributions coincide. If the equation is used beyond this approximation to generate nonlocal and density corrections, we conclude that the results are not necessarily correct.     

The spectrophotometric determination of nitrate in natural waters,with particular reference to sea-water

Nitrate can be reduced to nitrite in good yield by means of hydrazine in alkaline solution; the reaction is promoted by catalytic quantities of copper. The authors have established the optimum conditions for tlie reduction and applied the method to the determination of nitrate in fresh waters and in sea-waters. The nitrite formed is determined by Mellon and Rider's modification of the Griess-Ilosvay procedure. The reduction with hydrazine is carried out in the presence of 0.25 p.p.m. of copper at pH 9.6 in a solution buffered with sodium phenate. It is complete within 24 hours at room temperature. The method will detect ca. 0.3 μg NO₃$\text{N}\text{l}$ and gives a standard deviation of ca. 2% in the range 20-600 μg NO₃-$\text{N}\text{l}$. Up to 60 determinations can be made per 6 hour working period. The interference of nitrite has been investigated. Ammonium salts, urea, and amino acids do not interfere, at the concentrations at which they occur in sea-water.It is preferable to analyse samples immediately after collection, but if this is not possible, they should be filtered, sterilized with 2 p.p.m. of mercuric chloride and stored in glass containers.